The present invention relates to organic light-emitting diode devices and methods for making such devices, which use an organic buffer layer and a sputtered metal or metal alloy layer on such organic buffer layer.
Organic electroluminescent (OEL) device, alternately known as organic light-emitting diode (OLED), is useful in flat-panel display applications. This light-emissive device is attractive because it can be designed to produce red, green, and blue colors with high luminance efficiency, operable with a low driving voltage of the order of a few volts and viewable from oblique angles. These unique attributes are derived from a basic OLED structure comprising a multilayer stack of thin films of small-molecule organic materials sandwiched between an anode and a cathode. Tang et al. in commonly assigned U.S. Pat. Nos. 4,769,292 and 4,885,211 have disclosed such a structure. The common electroluminescent (EL) medium is comprised of a bilayer structure of a hole-transport layer (HTL) and an electron-transport layer (ETL), typically of the order of a few tens of nanometer (nm) thick for each layer. The anode material is usually an optically transparent indium tin oxide (ITO) film on glass, which also serves as the substrate for the OLED. The cathode is typically a reflective thin film. Selection of electrode materials is based on work functions. ITO is most commonly used as the anode because it has a high work function. Mg:Ag alloys are generally used as electron-injecting contacts because they have lower work functions. Lithium containing alloys such as Al:Li and LiF/Al contacts also provide efficient electron injection. The device emits visible light in response to a potential difference applied across EL medium. When an electrical potential difference is applied at the electrodes, the injected carriers-hole at the anode and electron at the cathode-migrate towards each other through EL medium and a fraction of them recombine to emit light.
In the fabrication of OLED devices, vapor deposition methods are often used. Using these methods, the organic layers are deposited in thin-film forms onto ITO glass substrates in a vacuum chamber, followed by deposition of the cathode layer. Among the deposition methods for the cathode, vacuum deposition using resistive heating or electron-beam heating has been found to be most suitable because it does not cause damage to the organic layers. However, it would be highly desirable to avoid these methods for fabrication of cathode. This is because these processes are inefficient. In order to realize low cost manufacturing, one must adopt and develop a proven robust high-throughput process specific to OLED fabrication. Sputtering has been used as a method of choice for thin film deposition in many industries. Conformal, dense, and adherent coatings, short cycle time, low maintenance of coating chamber, and efficient use of materials are some of the benefits of sputtering.
Sputtering is not commonly practiced for fabrication of the OLED cathodes because of the potential damage inflicted on the organic layers and the degradation of device performance. Sputter deposition takes place in a high energy and complex environment that is comprised of energetic neutrals, electrons, positive and negative ions, and emissions from the excited states that can degrade the organic layers upon which the cathode is deposited.
Liao et al. (Appl. Phys. Lett. 75, 1619 [1999]) investigated using x-ray and ultraviolet photoelectron spectroscopies the damages induced on Alq surfaces by 100 eV Ar+ irradiation. The core level electron density curves show that some Nxe2x80x94Al and Cxe2x80x94Oxe2x80x94Al bonds in Alq molecules were broken. The valance band structure was also tremendously changed implying the formation of a metal-like conducting surface. It was suggested that this would cause nonradiative quenching in OLEDs when electrons are injected into the Alq layer from the cathode and also would result in electrical shorts.
During sputter deposition of the cathode, the Alq surface is subjected to high doses of Ar+ bombardments at several hundred volts. As shown by Hung et al. (J. Appl. Phys. 86, 4607 [1999]), a dose only of 9xc3x971014/cm2 alters the valance band structure. Therefore, sputtering a cathode on Alq surface in Ar atmosphere is expected to degrade the device performance.
Sputtering damage is controllable, to some extent, by properly selecting the deposition parameters. European patent applications EP 0 876 086 A2, EP 0 880 305 A1, and EP 0 880 307 A2, Nakaya et al., of TDK Corporation, disclose a method of depositing a cathode by a sputtering technique. After depositing the organic layers, with vacuum still kept, the devices were transferred from the evaporation to a sputtering chamber wherein the cathode layer was deposited directly on the electron-transport layer. The cathode was an Al alloy comprised of 0.1-20 a % Li that additionally contained at least one of Cu, Mg, and Zr in small amounts, and in some cases had a protective overcoat. The OLED devices thus prepared used no buffer layer and it was claimed to have good adhesion at the organic layer/cathode interface, low drive voltage, high efficiency and exhibited a slower rate of development of dark spot. Grothe et al. in Patent Application DE 198 07 370 C1 also disclosed a sputtered cathode of an Al:Li alloy which had relatively high Li content and having one or more additional elements chosen from Mn, Pb, Pd, Si, Sn, Zn, Zr, Cu, and SiC. In all of those examples no buffer was used, yet electroluminescent was produced at lower voltages. Sputtering damage was controlled possibly by employing a low deposition rate. By lowering sputtering power it is expected that damage inflicted on the organic layers can be reduced. At low power, however, the deposition rate can be impracticably low and the advantages of sputtering are reduced or even neutralized.
To minimize damage during high speed sputtering of cathodes, a protective coating on the electron-transport layer can be useful. The protective layer, alternately termed as the buffer layer, must be robust to be effective. However, in addition to being resistant to plasma, a buffer layer must not interfere with the operation of the device and must preserve the device performance. Parthasarathy et al. (Appl. Phys. Lett. 72, 2138 [1998]) reported an application of a buffer layer consisting of copper phthalocyanine (CuPc) and zinc phthalocyanine (ZnPc) during sputtering deposition of a metal free cathode. The buffer layer prevented damage to the underlying organic layers during the sputtering process. Hung et al. (J. Appl. Phys. 86, 4607 [1999]) disclosed the application of CuPc buffer layers that permitted high-energy deposition of alloy cathodes. The cathode contained a low work function component, Li, which was believed to diffuse through the buffer layer and provided an electron-injecting layer between the electron-transport layer and the buffer layer. EP Patent Application 0 982 783 A2, Nakaya et al., disclosed a cathode of Al:Li alloy. The cathode was prepared by sputtering using a buffer layer constructed of a porphyrin or napthacene compound that was deposed between the electron-transport layer and the cathode. The device containing the sputtered electrode exhibited low drive voltage, high efficiency, and retarded dark spot growth. Although efficient devices based on sputtered cathode were disclosed, much was desired to simplify the materials and processes. For example, cathode generally was sputtered from an Al alloy target that contains an alkali metal e.g., Li. With this approach the target itself is the source of the electron-injecting dopant. Due to dissimilar properties particularly with respect to melting point, vapor pressure and other properties of alkali metals and other component metals, fabrication of homogeneous and quality target could be quite difficult. It is desirable to use pure metal, as high quality targets are readily available.
It is therefore an object of the present invention to provide an improved OLED device structure for injecting electrons either into the electron-transport layer or directly into the emissive layer.
It is another object of the present invention to facilitate the use of sputtering of the cathode layer.
The above objects are achieved in an OLED device, comprising:
a) a substrate;
b) an anode formed of a conductive material disposed over the substrate;
c) an emissive layer having an electroluminescent material provided over the anode layer;
d) a buffer layer, provided over the emissive layer and including phthalocyanine or derivatives thereof;
e) an electron injecting dopant source layer provided over the buffer layer and including a compound of an alkali metal or thermal decomposition products thereof; and
f) a sputtered layer of a metal or metal alloy provided over the buffer layer and selected to function with the buffer layer to inject electrons into the emissive layer.
The present invention provides a device structure for an OLED device that offers significant protection against damage during sputtering deposition of cathode layer.
An advantage of the present invention is that damage to organic layers during sputtering is minimized, permitting cathode fabrication at high deposition rates.
Another advantage of the present invention is that the sputtered layer does not require to be an alloy containing an electron injecting dopant.
Another advantage of the present invention is that the sputtered layer can be of pure metal not necessarily of low work-function.
Another advantage of the present invention is that it permits the use of wider manufacturing tolerance.